A conventional photoelectric conversion device includes a transparent substrate, a light source joined to the reverse surface of the substrate, a photoelectric conversion layer, and electrodes provided above the photoelectric conversion layer.
In operation, light projected toward a copy surface of a paper passes through the substrate, and is reflected from the copy surface to the detecting surface of the photoelectric conversion layer. In response to the light received by the photoelectric conversion layer, a corresponding electric signal is generated. When the light is projected toward the paper, part of the light may go toward the reverse surface of the photoelectric conversion layer, which phenomenon should be prevented.
According to this conventional photoelectric conversion device, the gate electrode is used as a light shield layer to shade the light going toward the reverse surface of the photoelectric conversion layer. The gate electrode can be fabricated in the same processing as of the sensor (photoelectric conversion device) itself. However, charge-up condition may transiently occur at the gate electrode when photoelectric charge is generated during the photoelectric conversion. Therefore, capacitance is generated between the gate and source electrodes and the gate and drain electrodes, especially, an photoelectric charge based on the potential of the gate electrode may be accumulated on the surface of the MIS transistor. The accumulated photoelectric charge makes the level of output signal lower and response time slower.
Accordingly, an improved photoelectric conversion device has been described in Japanese Patent Publication Kokai S63-161683, in which gate and drain electrodes of the transistor are short-circuited to each other to keep the potential between the gate and a source stable. According to this photoelectric conversion device, no parasitic capacitance exists between the gate and drain, however, some parasitic capacitance exists between the gate and source; and therefore, the ".gamma.-value" which is the gradient of logarithm of current vs logarithm of light intensity logalithm is reduced, and quick response has not been realized by the technique yet.